INTRODUCTION 



Variation. 



Probably variability is an original characteristic of all 

 organisms ; but the laws of inheritance, and the favouring 

 of a particular type by natural selection, have tended to pro- 

 duce constancy. This constancy is, however, often imperfectly 

 realised ; that is to say, all insects show more or less variation 

 in some particulars. Thus there is always some variability in 

 size and proportions ; often also in colour and markings. Varia- 

 tions in structure are less common ; in certain groups there is 

 variability in some details of neuration. Allied species may be 

 expected to vary in the same way, but otherwise experience 

 is the only guide, the same characters which are absolutely 

 constant in one group being often highly variable in another. 



Varieties may be broadly classified as being of two kinds, 

 viz. gradual and sudden ; the former being connected with 

 the type-form by numerous slight intermediate gradations, 

 the latter not so connected. The former kind represent the 

 outcome of those subjective tendencies to variation which 

 have not been eliminated by the working of natural selection, 

 and may be termed normal ; whilst the latter are apparently 

 due to some objective interference with the process of develop- 

 ment, and are in that sense abnormal. The line between these 

 two classes is not always easy to draw in practice, but the 

 distinction is real. 



Gradual varieties attached to a particular locality or region 

 are called local or geographical forms; those occurring at a 

 particular time of year seasonal forms. Variation caused by 

 the failure of colouring matter is termed albinism ; that due to 

 an excessive prevalence of black pigment is melanism ; but the 

 application of these terms to varieties which are merely lighter 

 or darker than usual is erroneous. Specimens showing sudden 

 variations in colour or marking are commonly termed aber- 

 rations; in structure, monstrosities. The most common examples 

 of monstrosities are produced by multiplication of parts, as when 

 an insect possesses an additional leg or vein ; or by gynandro- 

 morphism, the term used when an individual of one sex exhibits 

 on one lateral half the organic characters of the other sex, more 

 or less completely. 



Principles of Classification. 



It is now admitted that the resemblances of allied genera 

 and species are to be explained by community of descent. 



